1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) receiver, an OFDM reception method and a terrestrial digital receiver. In particular, the present invention relates to an OFDM receiver, an OFDM reception method and a terrestrial digital receiver to which antenna diversity is applied.
2. Description of the Related Art
The modulation method employed in terrestrial digital broadcasting is Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a form of a multi-carrier method or, in other words, a modulation method for transmitting information using a large number of carriers. Compared to a single-carrier method, OFDM is less influenced by transmission lines (particularly, multipath). OFDM has a buffering period called a guard interval in the head portion of a symbol (a single unit of transmission information). OFDM is considered to be multipath-resistant in this respect, as well.
However, the functions inherent to OFDM (the multi-carrier and the guard interval) are insufficient for a mobile-type OFDM receiver that is likely to be used in a severe environment, such as in a vehicle traveling at high speeds. Therefore, other anti-multipath techniques are used in combination. Typically, antenna diversity is applied.
As an example of an OFDM receiver to which antenna diversity is applied, a technology described in Japanese Laid-Open Patent Publication No. 2003-229830 is known (hereinafter, referred to as conventional prior art). In the conventional prior art, a plurality of antennas receive OFDM signals. The conventional prior art determines a correlation value between each OFDM signal received by each antenna that has been down-converted to the IF band and a delay OFDM signal that is delayed from each OFDM signal by an amount equivalent to a single effective symbol. Then, the conventional prior art calculates a carrier-to-noise (C/N) ratio (a ratio of additional noise power and signal power at a reception point) from the correlation value. The conventional prior art selects an equalization carrier signal with the highest C/N ratio among carrier signals of the same number obtained from each branch circuit and decodes the selected carrier signal.
As described above, the conventional prior art “determines a correlation value between each OFDM signal received by each antenna that has been down-converted to the IF band and a delay OFDM signal that is delayed from each OFDM signal by an amount equivalent to a single effective symbol and calculates a carrier-to-noise (C/N) ratio from the correlation value”. Briefly stated, the conventional prior art is interpreted to be “using information of the guard interval”.
FIG. 8A is a conceptual diagram of the guard interval in the conventional prior art. Three temporally consecutive symbols (K−1, K, and K+1) are considered, as shown in FIG. 8A. K denotes a current symbol. K−1 denotes a temporally preceding symbol. K+1 denotes a temporally subsequent symbol. The lengths of individual symbol periods T are fixed. For example, the length of the symbol period T in terrestrial digital broadcasting is 1.008 μs (in Mode 3). All symbols include an effective symbol period Tu storing transmission information and a guard interval period Tg (hatched portion) of a constant length added to the head portion of the effective symbol period Tu. A portion (end portion) of the information stored in the effective symbol period Tu subsequent to the guard interval period Tg is copied to the guard interval period Tg of each symbol.
FIG. 8B is a conceptual diagram of a use of the guard interval information in the conventional prior art. In FIG. 8B, a non-delay symbol and a delay symbol are the same symbol (for example, symbol K). The delay symbol is the symbol K delayed by a predetermined amount of time (T−Tg). The delay symbol is equivalent to the “delay OFDM signal” in the conventional prior art. When the correlation between the two symbols (the non-delay symbol and the delay symbol) is evaluated, the evaluated value is a large value during an overlapping period (overlapping periods Y and Z) of the end portion of the effective symbol of the non-delay symbol and the guard interval symbol period Tg of the delay symbol, because the information in Y and Z is originally the same.
The conventional prior art is interpreted to be a technology that evaluates the correlation between the information in Y and Z, and selects and decodes the equalization carrier signal with the highest C/N ratio, based on the principle described above.
However, the following two points can be pointed out regarding the conventional prior art. First, diversity in the conventional prior art is equivalent to a so-called composition diversity. In composition diversity, the signal having the best C/N ratio is selected from among the signals received by a plurality of antennas. Although composition diversity is superior in performance to a method in which a plurality of antennas are switched (selection diversity), composition diversity has a drawback of increased hardware size. Therefore, problems such as an increase in device weight, an increase in power consumption, and an increase in manufacturing cost are unavoidable.
Second, the conventional prior art evaluates the correlation only during the guard interval period Tg. In other words, the conventional prior art does not evaluate the correlation during other periods (effective symbol period Tu). Therefore, if, for example, the state of the transmission line suddenly changes during the effective symbol period Tg, the conventional prior art cannot immediately respond to the change. As a result, instantaneous symbol loss may occur. This drawback cannot be overlooked in an OFDM receiver that may be used in a vehicle traveling at high speeds, because the characteristics of the transmission line changes significantly during a short amount of time corresponding to the traveling speed.